Field of the Invention
The present invention relates to an electric power system capable of transmission and reception of electric power to and from an electric vehicle.
Description of the Background Art
Today, in an electric vehicle which uses electric power charged in a battery as a drive source for driving a motor to thereby obtain power, the battery can be charged by a household power supply through a cable. In recent years, more and more homes employ private power generation using photovoltaic power generation or wind power generation. In a case where the homes have any surplus electric power, the surplus electric power is sold to an electric power company, and in a case where the amount of power generation is insufficient, the homes use commercial electric power (system electric power) supplied from an electric power supply system of an electric power company. Advanced version of such a system is a next-generation electric power network called a smart grid.
In the current situation where homes are introducing electric power supply based on private power generation, a battery of an electric vehicle can be charged with electric power obtained by photovoltaic power generation or wind power generation.
A configuration of charging a battery of an electric vehicle with electric power obtained by photovoltaic power generation is disclosed in, for example, Japanese Patent Application Laid-Open No. 1996-19193.
Japanese Patent Application Laid-Open No. 1996-19193 discloses: a household power conditioner for converting DC electric power generated by a solar cell module into AC electric power and supplying the AC electric power to a domestic load; and a system for re-converting AC electric power supplied from the power conditioner into DC electric power and storing the DC electric power in a battery of a gasoline vehicle or an electric vehicle, or converting the stored electric power into AC electric power and supplying the AC electric power to the household load.
Japanese Patent Application Laid-Open No. 1996-19193 does not disclose an internal configuration of the power conditioner, and therefore it is not clear what kind of power supply is used for controlling a control circuit necessary for the DC conversion and the AC conversion in the power conditioner. Japanese Patent Application Laid-Open No. 1996-19193 discloses that in a case where the supply of system electric power is stopped for a long time, a control mode of the power conditioner is switched to a voltage control mode to establish an autonomous operation of the power conditioner. Here, in either of the cases where the power supply to the control circuit in the autonomous operation is obtained from the vehicle battery and obtained from the solar cell module, the need for some voltage step-down means is expected, in order that the power supply to the control circuit which is approximately a few volts can be obtained from a high voltage of 100 volts or more. However, performing a step-down with a large input-output difference causes a problem of an increased switching loss and an increased switching noise.